Subject of the invention are thermally conductive thermoplastic compositions having balanced processing properties, comprising a thermoplastic, a particulate, thermally conductive filler, at least one phosphorus compound, and also ethylene/alkyl (meth)acrylate copolymer.
The use of thermally conductive fillers for producing thermally conductive thermoplastic compositions is known. In order to give the thermoplastic composition a thermal conductivity sufficient for numerous applications, the thermoplastics are admixed with large amounts of thermally conductive fillers, which in some cases are platelet-shaped. This results in reduced processing properties of the modified particle composition, this being manifested in a significant reduction in the melt volume flow rate (MVR) of the thermoplastic compositions and hence in no longer complete filling of the respective cavities of moulds, even with short flow pathways. Moreover, the modulus of elasticity of highly filled thermoplastic compositions is too high—that is, they exhibit high resistance under elastic deformation acting from outside.
Thus US 2012/0319031 A1 describes compositions containing 30% to 90% of an amorphous thermoplastic or of at least one semi-crystalline thermoplastic, or a mixture of the two, and 10% to 70% of an expanded graphite, with about 90% of the particles of the expanded graphite having a particle size of at least 200 μm. Disadvantages of these compositions are the inadequate flow capacity and also the high modulus of elasticity, allowing the compositions described to be used only in selected applications.
WO 2009/043850 A2 describes heat-processable, thermally conductive polymer compositions containing 30 wt % to 95 wt % of a thermoplastic polymer and also 5 wt % to 40 wt % of a graphite powder in the form of platelets having a thickness of less than 500 nm, where the graphite powder preferably has a specific BET surface area of at least 10 m2/g as determined by the method of ASTM D3037, and a particle size distribution, determined by laser diffraction, that is characterized by a D(v, 0.9) of at least 50 μm. The compositions described in that document achieve low thermal conductivity and can be processed only to a limited degree in case of prolonged flow pathways. Moreover, the particle size distribution of the preferred graphite powders is too low for any significant improvement in thermal conductivity to be achieved.
WO 2010/061129 A1 describes thermoplastic polymer compositions comprising special expanded graphites having a BET surface area of 15 m2/g to 30 m2/g, with a bulk density of less than 0.1 g/cm3 and also with an average particle size expressed by a D(v, 0.5) of greater than 15 μm. On account of the fine graphite particles of low bulk density, the compositions modified therewith are not sufficiently processable in relatively long cavities. According to the values stated in this document, the thermal conductivity achieved using these compositions is, for many applications, too low.
JP 2007-031611 A describes thermoplastic compositions containing 20 to 99 parts by weight of a thermoplastic polymer and also 1 to 80 parts by weight of a graphite distinguished by a bulk density s 0.15 g/cm3. The graphite is further characterized by the pH of a specific suspension of graphite in water. The document provides no information about the processing properties or the heat distortion resistance of the resultant compositions. In relatively long cavities it is not possible to process the compositions claimed, which contain more than 45% of graphite.
JP 2011-178889 A describes compositions for LED lighting holders, the compositions comprising 40 to 95 parts by weight of a thermoplastic polymer and 5 to 60 parts by weight of a thermally expanded graphite, with 2 to 25 parts by weight of a phosphorus-based flame retardant additive and 0.01 to 1 part by weight of a fluorine-containing anti-dripping agent, based on the total amount of polycarbonate and expanded graphite. Specimens produced from the composition and having a thickness of 1.8 mm receive a UL 94 assessment of V0 and at the same time have a thermal conductivity of 4 W/(m*K). Use of the fluorine-containing anti-dripping agent in the compositions described results in a distinct reduction in the heat distortion resistance. Compositions with more than 15 parts by weight of the phosphorus-based flame retardant additive likewise exhibit a marked reduction in heat distortion resistance, and the melt volume flow rate (MVR) also falls in such a way that processing by injection moulding is hindered.
Consequently, there are no known compositions of high thermal conductivity in the prior art that exhibit a balanced proportion of melt volume flow rate (MVR) and heat distortion resistance and which can therefore be processed flawlessly to component parts in relatively long cavities. There is also a lack of materials which exhibit low longitudinal shrinkage and which exhibit sufficient resistance to elastic deformations applied from outside, without exhibiting excessive rigidity.